When giving an IME response, if you type part of the word / some information, then move the cursor backwards to add a letter. After you enter something the IME system registers, it moves your cursor to the end of the input field. Requiring you to move your cursor back to the original position if typing more than one letter.
As far as I know, WaniKani still uses their open source library WanaKana for their IME logic. I submitted a bug for this on August 8th, 2017 and it was fixed on October 11, 2017. Based on this, WaniKani probably just has to update to at least version 2.3.1 to fix this (latest version is 2.3.4).